Density Of Microfilariae Research Articles

Seasonal variation in the microfilarial skin density of Onchocerca volvulus and in the biting activity of Simulium spp. in Guatemala

The density of Onchocerca volvulus microfilariae in the skin of 11 carriers was examined by carrying out a monthly examination in a single endemic area of Guatemalan onchocerciasis. The results obtained suggested the possibility of a seasonal variation in the microfilarial skin density. Almost all of the volunteers participating had shown a peak concentration of microfilariae in September, followed by August or October. The highest biting densities of Simulium ochraceum, the principal vector of onchocerciasis in Guatemala, occurred from September to the following January. It thus appears that the seasonal variation in the microfilarial density may correspond, to some extent, with the biting activity of the vector, S. ochraceum, and/or with climatic changes in the area.

The maximum density of anopheline mosquitoes that can be permitted in the absence of continuing transmission of filariasis

Using two years of mosquito collections from two fixed stations on Guadalcanal it was possible to compare the average man-biting rate with the prevalence of microfilaraemia. One area was found to be free of infection while the other still had one positive case. The DDT residual spraying had caused a change in biting habits of A. farauti No. 1 allowing a greater density of mosquitoes to bite man at times when they had lower microfilaria densities. This difference was compared with the pre-spray density so that the maximum permissible number of mosquitoes biting man without transmission of filariasis taking place, could be calculated. A theoretical value for m of 19.84 had been worked out previously, this study gave a corrected observed value of m of approximately 20, very close to the calculated theoretical value. The situation studied here probably showed incomplete interruption of transmission, so it was considered preferable to recommend a figure of 0.66 average vector man-biting rate per hour indoors for the critical value.

The distribution of microfilariae in the skin of Guatemalan onchocerciasis patients: an evaluation of diagnostic potentials.

Quantitative examinations for the distribution and density of microfilariae were carried out on male Guatemalan onchocerciasis patients by taking 6 or 13 skin biopsies each from 74 subjects. We found that microfilariae were distributed most frequently on the iliac crest and secondarily in the scapular region; higher detection rates and greater microfilarial densities were seen in these areas. From these results we recommend the taking of biopsies from both of these anatomical regions so as to avoid false negative cases. The infected patients were classified into 3 groups (light, moderate and heavy infections) on the basis of the numbers of negative skin snips out of the 13 biopsies taken. In heavy infections, a higher densities of microfilariae were detected in the head and neck regions, as compared with the densities in the lower extremities (calf). The high concentration of microfilariae in these regions is of importance in relation to the development of eye lesions and/or transmission. Of patients who had microfilariae in the anterior segment of the eye some had negative skin biopsies frm the outer canthus, retroauricular region and neck.

GUATEMALAN ONCHOCERCIASIS : SKIN SNIPPING METHODS AND MICROFILARIAL DENSITIES IN A GIVEN MINUTE AREA OF THE SKIN

To establish a convenient and reliable method for diagnosis and epidemiological surveys on Guatemalan onchocerciasis, three skin biopsy instruments were evaluated in 108 volunteers. The German-made Holth type, the Japan-made Walser type corneoscleral punches and a lancet and disposable scalpels were used for the purposes of taking skin snips. Better results were obtained with the lancet and scalpel, indicating a higher detection rate or a lower false negative rate for the patients. This method, however, showed some disadvantages in the present study. The volunteers had various complaints and/or rejected our examinations because of the painfulness of the scalpel procedure. On the other hand, the Holth punch was better than the Walser punch which indicated a relatively high false negative case and was rather troublesome in handling. From the results obtained, we recommend the Holth punch as the skin snip apparatus in Guatemala. To examine the microfilarial densities in a given minute area of the skin, six skin snips each (12 snips in total), 0.5 cm apart, were taken by the Holth punch from 33 subjects. In this study, there were considerable numbers of negative skin snips, especially in those patients who had a relatively low density of microfilariae. This would be of importance for diagnosis and epidemiological surveys for Guatemalan onchocerciasis. The results seem to indicate that if one skin snip was taken from the patients of a low endemic area in Guatemala, the prevalence of onchocerciasis in the area might be underestimated with false negative skin snips. Moreover, there was a great variation in the microfilarial densities in the 12 skin snips from each subject. The maximum to minimum rate of microfilarial densities in average was 1 : 9.7 in the left scapula and 1 : 7.1 in the left iliac crest of the 33 subjects. These facts offer some useful suggestions for us, when we examine the intensity of infection in an endemic area or investigate a diurnal or seasonal variation of microfilarial densities of Onchocerca volvulus in Guatemala.

Immunologic response in onchocerciasis: relations to microfilaria density in the skin biopsy and to some frequent symptoms.

During physical and immunologic investigations on onchocerciasis in 1,298 persons in the Republic of Liberia, the following results were obtained. There is a significant correlation between immune response and Onchocerca volvulus infection detected by a microfilaria-positive skin biopsy. On the other hand, the immune response is not influenced by the number of parasites in the skin, and there is no correlation of the immune response with Onchocerca nodules and with pruritus. There is a high proportion of serologically false-positive and false-negative results, and thus a diagnosis cannot be made simply by establishing evidence for antibodies in one person.

The prevalence and distribution of Mansonella ozzardi in coastal north Trinidad, W.I.

A day blood smear survey for Mansonella ozzardi in the north coast communities of Trinidad recorded a prevalence of 4·8% in 4,488 persons examined. Prevalence rates were highest in the four westernmost communities, where the vector, Culicoides phlebotomus, is a severe nuisance problem. Prevalence rates increased with age and were higher in males than in females. Mean microfilaria densities were low in both sexes up to 50 years of age but in older males the density increased with age. Wuchereria bancrofti infections were detected in five of the ten communities surveyed.

Bancroftian filariasis in Puerto Limón, Costa Rica.

In four neighborhoods of Puerto Limón, a Caribbean coastal city in a tropical rainforest area of Costa Rica, examination of 1-ml samples of night blood from 1,142 randomly selected, and 1,196 associated, persons by the Knott and filter-chamber techniques revealed microfilariae of Wuchereria bancrofti in about 3% of the 2,338 samples. The frequency of infection was higher in males (3.9%) than in females (1.9%), higher in persons of black (4.1%) than of white (1.0%) race, and highest in persons aged 10-19 (4.0%) and over 50 years (4.8%), lowest in those under 10 years (1.1%). The median microfilaria density was 3.5, the highest 45, per 20 lambdas of blood. Microfilaremia was distinctly periodic. Dissection of 663 female Culex pipiens fatigans from 42 houses of infected persons revealed filarial larvae in 25; only 1 larva was third (infective) stage. Of 64 infected persons, 11 had clinical findings suggestive of filariasis. Elephantiasis was seen in 21 others. Other forms of symptomatic filariasis without microfilaremia, though presumed to be present, were not assessed specifically.

Observations on the basal follicle numbers developed per female of two strains of Aedes aegypti after being fed on hosts with different levels of microfilariae of Brugia pahangi

Results indicate that there is a significant correlation between the density of Brugia pahangi microfilariae on which Aedes aegypti feed, the mean number of infective larvae produced per mosquito, and the mean number of basal follicles developed per female. When the density of microfilariae in cat's blood was high, the mean number of basal follicles developed per female was proportionally lower. It is concluded that the infected mosquitos show a significant drop in numbers of basal follicles produced per female. Both in the susceptible and refractory mosquito strains infected with B. pahangi, there was a negative linear correlation between parasite load and the number of basal follicles developed per female following their infecting blood meal.

Onchocerciasis in Guatemala

Periodicity of microfilariae of Onchocerca volvulus in the skin was not believed to exist until investigators in Africa recently demonstrated diurnal periodicity. Interestingly, this maximal density of microfilariae in the skin has been shown to coincide with the peak biting time of the Simulium vector. The current study was performed as part of other studies in Guatemala. Maximal density of microfilariae in the skin was at 1000 hours or shortly therafter. This peak is earlier than reported in Africa. The most active feeding period of Simulium ochraceum, believed to be the principal vector in Guatemala, had been previously shown to be from 0800 to 1000 hours. The current report lends support to other findings indicating that Onchocerca volvulus microlilariae have a diurnal periodicity. In addition, the occurrence of peak vector biting time at the same time as maximal numbers of microfilariae in the skin strengthens a hypothesis that these synchronous cycles have a biological significance.

Evaluation of mass treatment of malayan filariasis by diethylcarbamazine in Cheju Island.

Control programme of malayan filariasis by diethylcarbamazine was set up in village, Cheju-Island, Korea. The daily dose of 6 mg per kg of body weight for 6 days was chosen as a chemotherapeutic course only for microfilaria positives, which was repeated with an interval of one or two months. For the evaluation of drug control programme the yearly posttreatment blood survey was carried out. The results collected from 1970 to 1973 were analysed from the various points of view, such as microfilaria positive rates, incidence as well as prevalence rates. The change of microfilaria counts was also utilized for the comparison of the intensity of infection among population in the four year blood surveys. Considering the extremely skewed distribution of average microfilaria density, the mode of frequency distribution of microfilaria density in a population was particularly taken into account for the evaluation of changing endemicity affected by diethyecarbamazine treatment. The equation; Y = a + b log X(a, b; constants, X=microfilaria density; Y=probit scale of cumulative frequency of positives to X microfilariae) was found to fit farily well the data collected from four-year surveys before and after treatment. From these data obtained, four regression lines have been drawn by determining "a" and "b"; two paramenters, which were increased year by year since inauguration of control programme. In the present investigation, it was verified that the chmotherapeutic control measure of malayan filariasis applied in a village of Cheju-Island has been successfully carried out and the quantitative level of endemicity inthis area has also been distinctly lowered within the four-year control programme.

Effectiveness of diethylcarbamazine in the mass treatment of malayan filariasis with low dosage schedule

A series of 250 patients with filariasis due to periodic type of Brugia malayi was selected for the mass treatment with diethylcarbamazine from two areas in Cheju-Do. Two types of dosage schedules have been employed: the one was the conventional dosage schedule (uninterrupted dose of 6 mg/kg once daily 6 times repeated with an interval of one or two month totaling 72 mg/kg), and the other was the low dosage schedule (uninterrupted dose of 0.5, 1, 2, 4, 6 mg/kg body weight given once a day with an additional 6 mg/kg once daily totaling the maximum 37.5 mg/kg). The relative effectiveness of those two schedules was evaluated under the basis of the effects on microfilaria counts and the results of analysis of side-reactions observed. The results obtained in this experiment are summarized as it follows. In the conventional dosage treatment, 118 out of 141 microfilaria positives treated (83.7%) became negative and microfilaria reduction rate was about 99%. However, in the low dosage treatment, 35 out of 43 positives treated (85.4%) became negative and reduction rate of microfilaria, 99.5% in average. Total dose and duration of treatment were 72 mg/kg and about 60 days in the former schedule but it were maximum 37.5 mg/kg and 10 days in the latter. The recurrence of filariasis after treatment in both conventional and low dosage was negligible. Comparative analyses regarding the resulting side-reactions between the two schedules were made on their frequency, severity, onset and duration and relationship with microfilaria density. The febrile reaction was most frequently observed in 80.5% in the conventional, 43.9% in the low dosage schedule. The main side-reactions usually appeared within 6 to 10 hours and lasted 48 to 72 hours in the former threament, however they occurred in 7 to 8 hours and lasted 24 to 43 hours in the latter dosage schedule. It is assumed that the side-reactions are not directly related with the microfilaria count. However, the febrile reaction seems to be correlated with microfilaria density if it is above a certain limit. The presence of adult worm in a lymphnode involved with local reaction was ascertained from the biopsy specimens performed and local reactions such as lymphangitis and lymphadenitis after drug administrations were assessed from clinical and pathological point of view. It was conclusively indicated that the chemotherapeutic response to low dosage schedule within shorter length of treatment was equal to that of larger doses in long term medication from the point of the reduction in microfilaria, and particularly the resulting side-reactions were also reduced in the low dosage schedule.

Ecology of filariasis on Che Ju Island.

Study of filariasis to determine important factors involved in its ecology was carried out on Che Ju Island for three consecutive years from 1968 to 1970 in seven villages, three coastal villages and four islets remote from the main island. One village which was located in mountainous area far from the coast was surveyed to serve as control area. About 90% of population inhabiting the study area had at least one blood smear during the three-year period; about one third had three blood smears, and a little over one third had two, and the rest only one examination. Animal and mosquito surveys were carried out at the same period. Followings are the results obtained: 1. All human cases but several had microfilariae identical to the description of B. malayi. The several cases who had morphologically different microfilariae from that of B. malayi need further study for definite conclusion. 2. Five persons randomly sampled from Mf positives and bled every two hours demonstrated nocturnal periodicity between 9 p.m. and 3 a.m. 3. Human is considered to be only reservoir host for human filariasis in the area since animal survey and experimental exposure to the infective larvae of human filaria species showed failure to infect animals. 4. Microfilaria rate, microfilaria density, prevalence of elephantiasis varied by area and age with correlation, which indicated cumulative process of the parasite by repeated exposure and development of host immunity to certain extent. 5. Clinical manifestation of filariasis (symptom complex and elephantiasis ) taken from history and inspection was low in its prevalence with range of 0.9% 11.8% of total population. Only 5.2% of 517 Mf positives had the clinical manifestation. 24.8% of 109 persons with clinical manifestation had microfilaria; 42.9% with symptom complex only, 23.1% with both symptoms and elephantiasis, and none with elephantiasis only were microfilaria positive. 6. Ae. togoi was the only species infected with the filaria. Mosquito infection rate by area showed positive correlation to the Mf rate and density of human population; where the Mf rate and density were high, the mosquito infection rate also high.

The Pattern of a Host-Parasite Distribution

The pattern of density of the microfilaria Chandlerella quiscali among host specimens of the gnat Culicoides crepuscularis was fitted to the negative binomial distribution. Variations in the density of microfilariae in peripheral circulation of the grackle Quiscalus quiscula versicolor were suggested as the reason for the clumped density of nematodes among the gnats. Analyses of variations in pattern of distribution have been applied to plant and animal populations to gain insight to some of the biological interactions that vary over time and space: (e.g., Kershaw, 1964; Greig-Smith, 1964; Pielou, 1969; and Crofton, 1971a, b). Three general categories of pattern may be defined as (1) regular, (2) random and (3) clumped. Each category can be tested for fit by observed sets of distribution data, and each infers certain kinds of ecological phenomena. Deviations from the random (Poisson) pattern imply an interdependent relationship among individuals or between organisms and some environmental (habitat) variable. For example, regular patterns may be seen in species that saturate habitats to the level where minimum, equal distances separate all individuals. A clumped or contagious distribution pattern can be seen where habitats are not uniform and individuals are concentrated in the most suitable patches, or where interactions among individuals may hold organisms together in groups although the habitat is uniform. Analyses of parasite populations for variations in pattern of distribution among and within host specimens provide a means of defining certain categories of parasite-parasite or parasite-host interaction, and of extending our ability to describe symbiotic relationships in general. This report describes the analysis of pattern of distribution of microfilariae (Chandlerella quiscali) among host specimens of the gnat (Culicoides crepuscularis). MATERIALS AND METHODS The data for this analysis came from the study by Robinson (1971) on the role of the ceratopogonid gnat Culicoides crepuscularis as a host for the filarial nematode Chandlerella quiscali that Received for publication 31 March 1972. parasitizes the purple grackle, Quiscalus quiscula versicolor. In that study, 2 kinds of traps and a variety of trapping times were used to expose and collect the Culicoides. However, the data for the present paper include only 143 gnats from the total collection, that were taken under a single set of standard conditions. The same infected bird was used for all exposures. The microfilarial density in its peripheral blood during the hours of trapping was not determined, but relative to infections studied in many other birds this one was light to moderate. The trapping time was 2100 to 2300 hr, a period of the day in which it was known that gnats were maximally active, and in which the nocturnally periodic microfilariae were swarming in the peripheral blood of the host. Traps were run on nights in June and July. A modified funnel trap was used, in which 12 birds were contained as attractants, but the gnats could only reach the infected bird to feed. Three of these traps were run in different ecological situations and heights, known from previous work to be well populated by the gnats. Exposed gnats were separated from the unengorged ones, and maintained on honey water in small cages until killed for dissection and examination. There was no detectible mortality among the gnats during the developmental period of the worms, and likewise no mortality among the nematodes. Unengorged gnats served as controls and none contained microfilariae. The large number of gnats, n = 143, pr vided a sample size sufficient for statistical treatment. The raw data were tabulated in three ways. First all of the 143 gnats sampled were treated as a set for calculation of a sample mean and variance. Second, only those 85 gnats containing nematodes were tested for fit to a truncated (the zero frequency class was not included) Poisson distribution. Finally, all 143 gnats were separated into frequency classes by the numbers of contained nematodes and statistical testing was done for fits to the Poisson and the negative binomial (clumped) distributions.

Endemic filariasis in Thakurgaon, East Pakistan.

Abstract Thakurgaon, an area of 645 km2 in Dinajpur District of East Pakistan, was surveyed to determine the prevalence and distribution of human filariasis. Enumeration of microfilariae and clinical manifestations was obtained from peripheral blood smears and physical examinations of 9,624 inhabitants. The microfilaria rate of 16.8% represented infection by Wuchereria bancrofti of nocturnal periodicity. The clinical manifestation rate of 10.1% consisted primarily of genital hydrocele and elephantiasis of the scrotum in the male population. Lymphedema and elephantiasis of the limbs were rarely observed in either sex. Microfilaria infection and clinical manifestation rates increased with ascending age group. An endemicity rate of 24.2%, and a median microfilaria density of 14.0 microfilariae per 20 mm3 of peripheral blood, indicated a moderately endemic focus of filariasis that serves as a reservoir of infection and constitutes a significant public health problem in Thakurgaon.

Studies on filariasis in Korea - On the morphology and development of larvae of Brugia malayi in Aedes togoi

Since Senoo and Lincicome (1951) first have brought up for attention to the existence of malayan filariasis in Korea, several reports on the epidemiological investigations of the disease had already been made by many workers. However it is little known what kind of mosquitoes are involved as the major vectors in main endemic areas. In Cheju-Do, known as one of main endemic areas in Korea, Aedes togoi is most likely suspected as an important vector because of their abundant collections and vigorous biting attack to human. As a part of studies on filariasis in Korea, an essential preliminary is to determiine whether this mosquito, Aedes togoi collected in the above areas is receptive to the microfilariae of B. malayi. Therefore, the present paper is concerned chiefly with the development of B. malayi in A. togoi. It is also hoped that the studies on the larval morphology in the mosquito host and the structure of microfilariae will provide the base line data required for later investigation of the different vector hosts. The studies were summarized as follows: 1)The measurements of the fixed points in percentage of the body length of microfilariae from the Giemsa stained thick films were made, and they showed that cephalic space was 8 %,cephalic space length to width, 1.3:1, nerve ring, 21.2 %, excretory pore, 30.8 %, excretory cell, 36.5 %, R1 cell, 66. 5 %, anus 80.4 % and body length 202 micro(l81-228 micro) maximun width 7.6 micro. 2)A study on the development of microfilaria malayi in the mosquito, Aedes togoi was carried out at room temperature (24-30 degrees C). Mosquitoes used in this experiment were reared from larvae collected from the tide water rock pool in the coastal areas of Cheju-Do and they were fed with a blood meal of carrier donors whose microfilaria densities were in the range from 0.5 to 0.7 per cmm of blood. 3)All of the microfilariae ingested by mosquito exsheathed in stomach, penetrated into the body cavity and then migrated into the thoracic muscles of the mosquitoes within 10 hours, after two moults in the mosquito host, the length of the developing 3rd stage larvae reached in size of 1.3~1.7 mm X 23~32 microns with anal ratio, 2.6 to 3.6. The first appearance of 3rd stage larvae in the mosquito host in this experiment was in 8th day after infection. The larvae were observed in the various cavities of mosquito, such as head, thoracic cavity, abdomen, halters, eye and legs. During the larval development in larval development in the host, the shortening of body length was first observed and then elnongation was followed until becoming 3rd stage larvae. 4)Aedes togoi was proved to be the most suitable host for this species of microfilaria malayi in the above endemic areas.

Relation of age, sex, and microfilaria density to treatment of sub-periodic filariasis with diethylcarbamazine.

Summary A study group of 175 mf carriers of varying ages and mf densities, were treated for 5 or 6 days at 6 mg/kg and again after 6 mo. with a total dose of DEC between 60 and 72 mg per kg of body weight and examined regularly for microfilariae by two 20 cmm samples of blood at 1 week, 6 months, and 2 and 3 years after the beginning of therapy. The frequency of adverse reactions to the first administration of DEC varied directly with the pretreatment mf density of the carriers and with the age group receiving treatment: carriers with initially low counts and in the younger age groups, 0 to 19 years, experienced significantly fewer reactions to DEC than those with higher counts or in the older age groups, suggesting that, whatever the nature of these reactions, they appear to be related to the intensity of the infection and to the length of exposure to mf products. No correlation could be found between age groups or sex distributions and the percentage of carriers who remained positive 1 week after therapy or several months or years later. A significant positive correlation, however, was consistently found between pretreatment mf density and percentage of carriers found positive at various intervals after treatment, in that the highest positive rates after treatment were found among the carriers with highest initial counts, indicating that the dosage of DEC may have to be gauged to the pretreatment mf density of the individual person or to the population at risk, in order to insure optimal therapeutic results.

Biological aspects in the transmission of Brugia malayi by Aedes aegypti in the laboratory.

Biological factors influencing the efficiency of the Liverpool strain of Aedes aegypti (L.) as a vector for sub-periodic Brugia malayi (Brug) were investigated. The mean weights of blood meal in 3 groups of A. aegypti fed on cats with B. malayi were 3.2, 3.3 and 4.0 mg. A. aegypti took in more microfilariae than expected when fed on cats with sub-periodic B. malayi and 1/10 took in less than expected. Microfilariae of B. malayi penetrated the stomach wall of A. aegypti , entered the haemocoele, then penetrated the thoracic tissue. Microfilariae were found in the thoracic muscles within 10 min of ingestion of blood meal with migration continuing for 10 hrs, although most of the microfilariae migrated within the 1st hr. Blood clotting started within 20–30 min after ingestion of blood meal, thus inhibiting the movement of microfilariae. The figures on the migratory rate of the microfilariae when plotted on a logarithmic time scale showed 2 symmetrical curves indicating an inherent pattern difference in the rate of migration. The meaning of these 2 can only be surmised presently. Development of larvae of B. malayi in A. aegypti was similar to that described by earlier workers in Anopheles sinensis Wiedemann and Mansonia dives (Schiner). In individual refractory A. aegypti , all the larvae perished after 48 hrs of development while in individual susceptible mosquitoes, a few survived to complete development of the infective stage. There was an increase in the number of infective mosquitoes when fed on cats with higher densities of microfilariae, although no proportional increase in the number of larvae were found per mosquito. Survival was reduced among the mosquitoes during the first 48 hrs after the infective blood meal when fed on cats with microfilaria densities of 5.0 per cmm and above. The results of preliminary experiments on the age of the mosquito and its ability to support development of the parasite were inconclusive.

Control of Bancroftian Filariasis in the Ryukyu Islands: Preliminary Results of Mass Administration of Diethylcarbamazine

Summary The etiologic agent of filariasis in the Ryukyu Islands is Wuchereria bancrofti. It is nocturnally periodic and is transmitted by Culex fatigans. A program aimed at the control of filariasis began in January 1965. The island of Miyako was selected as the initial target, and over 97 percent of the population was examined and, if necessary, treated with diethylcarbamazine. Use of this drug over a one-year period successfully converted 82 percent of treated microfilaria carriers and produced an impressive decline in the average microfilaria density per 30 cmm of blood. The method of examination and treatment is discussed and data are presented on the results of therapy by municipality, age, and sex and the implications of these results are analyzed. The medical control of filariasis was complemented by extensive vector control measures. The relative failure of this aspect of the program is included in the discussion.

Onchocerciasis in the Usambara Mountains, Tanzania: The disease, its epidemiology and its relationship to ocular complications

A focus of onchocercal infection has been found in the Usambara mountains in the region of the Zigi valley and Amani. The incidence appears to increase with age, and by middle age approximately three-quarters of the population sampled were infected. A detailed clinical, parasitological and ophthalmic study was carried out on 88 patients, examined during the earlier survey and found to be infected. Among 84 patients shown during the study to have microfilariae in their skin, corneal lesions were present in 23·8%, iritis in 7·14%, fundal lesions similar to those of senile choroidal sclerosis in 10·85%, and microfilariae in the anterior chamber of the eye in 16·5%. Pruritus was the commonest symptom complained of, being present in 83·3% of patients. Thickening of the skin was present in 28·6%, an excoriated rash with or without depigmented patches in 61·9%, presbydermia in 14·3%. Laxity of the skin of the groin, constituting either incipient or actual “hanging groin,” was present in 9·5%. Onchocercal nodules were encountered in 48·8%. The microfilarial densities in the series were high, the mean number of microfilariae per milligramme of skin being at the shoulders 29·6, at the buttocks 51·3, at the ankles 26·9. It is notable that the highest density of microfilariae was encountered at the buttocks instead of at the ankles as in a number of other reported studies. No statistically significant differences between microfilarial counts from the left and right sides of the body were encountered. Within the range of size of skin snips examined in this series, distribution in depth of microfilariae was uniform. No difference was found between density of microfilariae in skin taken from over bony prominences and from nearby soft tissue. This and the previous two points are clearly of practical importance in survey work. A statistically weak correlation was found between density of microfilariae in the skin and age of the patient. There is a tendency for microfilariae to increase very slowly if at all beyond the age of 20 years, suggesting the development of partial immunity to infection, or a change in personal habits or environment leading to an infection rate low enough to maintain but not increase microfilarial density. Counts from neither shoulders, buttocks nor ankles showed any statistically significant correlation with either anterior or posterior ocular lesions or freedom from ocular lesions, but patients with posterior lesions had heavier counts and were older than those with anterior. These observations, though they could result from chance, suggest that posterior segment lesions in the eye are associated with heavy infections borne for many years. Heavy and prolonged onchocercal infection may be one of many factors which can accelerate senile ophthalmic change.

Onchocerciasis in Guatemala a clinical and parasitological study with comparisons between the disease there and in East Africa

A clinical and parasitological study has been carried out in Guatemala by observers who had carried out a similar study, using similar methods, at Amani, Tanzania. Some quite large differences were encountered in the frequency of various clinical, including ophthalmological, features in the two series. These differences included a greater frequency of iritis and keratitis, particularly severe sclerosing keratitis, but not of fundal lesions, in Guatemala and a lower frequency of skin lesions especially those of the legs. Mal morado, erisipela de la costa and leonine facies were encountered exclusively in Guatemala and occurred among the more heavily infected subjects. Differences were also encountered in the density and distribution of microfilariae in the body of Guatemalan and African subjects. Microfilarial densities in the legs were low in Guatemala and those in the upper part of the body proportionally higher, but even the heaviest loads in Guatemala were rather lighter than those encountered in East Africa, and differences were not sufficient to account for the differing frequency of clinical features. Differences in density and distribution of microfilariae do not wholly explain the different incidence and severity of these clinical and ophthalmic findings. In particular, similar parasite densities are associated with more keratitis and iritis in Guatemala than East Africa. This could result from the two strains of parasite being fundamentally different or from differing responses to similar parasite loads by Guatemalans compared with East Africans. In the Guatemalan series iritis and keratitis were associated with significantly higher parasite loads than in those free from ocular lesions. In endemic regions of Guatemala more people in early age groups were found to be infected than in East Africa. Density of microfilarial load was found in Guatemala as in East Africa not to increase significantly after the age of 20, suggesting the development of immunity or a change in habits with increasing age leading to limitation of the number of infective bites sustained.